Fish and other seafood may contain beneficial nutrients as well as harmful contaminants. The prenatal period is a time of particular susceptibility to the adverse effects of organic mercury as well as to the beneficial influence of nutrients such as elongated n-3 fatty acids and selenium. However, little is known about the balance of risk and benefit from maternal fish intake during pregnancy on child development. The small number of previous studies regarding fish intake and child development are limited by lack of detailed nutrient measures, short duration of follow-up, and few outcome measures. In the proposed study, we will use information on maternal fish intake and biomarkers of mercury and nutrient exposure to examine the combined influence of the risks and benefits of maternal diet during pregnancy on child development. This project build upon the established infrastructure of Project Viva, an ongoing pre-birth US cohort study with stored maternal blood samples, validated prenatal dietary assessment, and detailed information on a number of covariates including home environment and other important predictors of child development. Study aims will be to assess associations of levels of mercury and n-3 fatty acids from maternal blood collected during pregnancy with child cognition and behavior at age 7 years. In addition, we will examine the influence of maternal prenatal fish intake on child cognition and behavior, with and without additional adjustment for mercury and n-3 fatty acids. We will explore the role of nutrients as modifiers of the effect of prenatal mercury exposure, by determining whether mercury has a stronger adverse effect among participants with lower levels of elongated n-3 fatty acids or selenium. Funds from this grant will support wide-ranging cognitive and behavioral assessments added to the already-funded cohort follow-up visit at age 7 years, as well as assays of exposure biomarkers and data analysis. This project is both cost- and time-efficient compared to the initiation of a new cohort, and offers many advantages over previous studies. The study team brings internationally recognized expertise in establishing, maintaining, and analyzing data from a longitudinal cohort, in performing and interpreting assays of nutrient and toxicant exposure, and in interpreting cognitive test results. The proposed study will provide information important to helping mothers and their health advisors make the best decisions about diet during pregnancy to optimize child development. The proposed study will provide timely and high quality data to inform the ongoing national policy discussion regarding the nutritional benefits as well as the contaminant risks from fish intake during pregnancy, and how these competing effects may balance out. Getting the message right is crucial, since pregnant women act upon recommendations regarding the safety of dietary fish intake. The right kind of message could improve children's developmental potential, but the wrong message could do the opposite.